1. Field of the Invention
This invention relates to lithography. In particular, the invention relates to extreme ultra-violet (EUV) lithography.
2. Description of Related Art
Extreme ultraviolet (EUV) lithography is an imaging technology with higher resolution capabilities than are available from longer wavelength exposure tools. The very short exposure wavelength ,requires an all reflective lens system because refractive materials are not sufficiently transparent. The EUV technique bounces EUV photons off a system of mirrors, including a mask made of reflective materials, that is ultimately focused on a resist-coated silicon wafer. By doing so, EUV systems can pattern features smaller than 0.05 micrometer.
Present EUV radiation sources have a very broad-band emission spectrum. For example, Xe laser pulsed plasma (LPP-Xe) sources have in excess of 40% of their radiation at wavelengths longer than 125 nanometers (nm) and approximately 60% of the energy is at wavelengths that are longer than 17 nm. EUV lithography tools for high volume manufacturing may require about 13.4 nm+/xe2x88x921% wavelength radiation. Power levels are expected to be in the range between 50 and 100 Watts. Thus, kilowatts of energy potentially need to be filtered from the source spectra. Otherwise, it would unreasonably distort the mask and mirrors used in EUV imaging tools.
Existing techniques include use of ultra-thin transmission filters based on coating of a membrane on support structures. This technique has low efficiency, typically passing only about 50% of the desired wavelength. In addition, the membrane filters are easy to rupture at high power levels because of absorption. Another technique uses mono-chrometer and diffraction of the actinic 13.4 nm light. This technique is substantially less efficient and is generally used with only synchroton sources. Yet, another technique uses cooled Mo/Si multi-layer coated mirrors. This technique operates only through absorption and selective reflection, leading to problems in heating and a lack of long wavelength filtering.
Therefore, there is a need to have an efficient technique for spectral filtering of power EUV radiation.